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Carbenium ion, 98

The best known stable carbocation salts are trijrfjenylmethylium salts, (C6Hs)3C A, initiating polymerization of certain olefins by direct additfon  [Pg.15]

The occurrence of the direct addition has been verified in some cases by the determination of the triphenylmethyl moiety in the polymer chain (UV, IR, NMR Trityl salts can easily be observed in solution because of their specific and strong absorption ( max 3-6 10 at Xmax 430 nm for (C6H5)3C A where A is a complex anion). [Pg.15]

The corresponding dissociation constants of trityl salts and related thermodynamic parameters are given in Table 2. [Pg.15]

In vinyl polymerization, the kinetics of initiation was studied by Sigwalt and Vairon for cyclopentadiene 4) and p-methoxystyrene using the spectrophoto-metric method. [Pg.15]

Stable trityl salts, which might lead to systems devoid of side reactions, do not however initiate polymerization of the majority of heterocyclic monomers by direct addition The only reported exception is the copolymerization of THF with propylene oxide  [Pg.15]


MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). [Pg.251]

This involves the formation of a carbenium ion which is best described as a hybrid of the two structures shown. This then rearranges by migration of a bond, and in so doing forms a more stable tertiary carbenium ion. Elimination of a proton yields camphene. [Pg.424]

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. [Pg.147]

German and French literature, indeed, frequently used the carbenium ion naming for trivalent cations. [Pg.149]

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. [Pg.149]

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). [Pg.362]

The Ritter reaction with unsaturated carbenium ions under either silver-assisted solvolysis or photolytic conditions leads to excellent yields of isoquiaolines (173). The ease of preparation of the requited vinyl bromides makes an attractive route to highly substituted isoquiaolines. [Pg.397]

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. [Pg.365]

The alkylate contains a mixture of isoparaffins, ranging from pentanes to decanes and higher, regardless of the olefins used. The dominant paraffin in the product is 2,2,4-trimethylpentane, also called isooctane. The reaction involves methide-ion transfer and carbenium-ion chain reaction, which is cataly2ed by strong acid. [Pg.370]

Donation of a proton to the reactant often forms a carbenium ion or an oxonium ion, which then reacts ia the catalytic cycle. For example, a catalytic cycle suggested for the conversion of phenol and acetone iato bisphenol A, which is an important monomer used to manufacture epoxy resias and polycarbonates, ia an aqueous mineral acid solution is shown ia Figure 1 (10). [Pg.162]

The second step is a -scission, the breaking of a carbon—carbon bond P to the charged carbon. The sum of the two reactions is the stoichiometry of the overall cracking reaction R H — RH + olefin. R+, a relatively stable carbenium ion such as the /-butyl cation, is a chain carrier. The role of the catalyst is to donate the proton to start the chain. This is a greatiy simplified representation. [Pg.179]

S-Substituted thiiranium ions react with water and alcohols to give trans ring opening (Scheme 72). A report that oxygen nucleophiles attack sulfur as well as carbon has been shown to be incorrect (79ACR282). The intermediate thiiranium ion (57) in the presence of lithium perchlorate readily yields the carbenium ion which undergoes a transannular hydride... [Pg.157]

S-Substituted thiiranium ions react with secondary amines to give ring-opened products. Nitriles also react with thiiranium ions, probably via an open carbenium ion whose formation is favored by increasing the polarity of the medium by the addition of lithium perchlorate (Scheme 79) (79ACR282). An intramolecular displacement by an amide nitrogen atom on an intermediate thiiranium ion has been invoked (80JA1954). [Pg.159]

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. [Pg.264]

Morphine (1) reacts with formaldehyde in acidic solution to yield a cyclic ketoalcohol (2) which is transformed into the colored oxonium (3) or carbenium ion (4) in acidic conditions [10],... [Pg.300]

TFA, CH2CI2, rt, 5-30 min, 84-99% yield." An adamantyl glycoside was stable to these conditions. The reaction has also been performed in the presence of anisole to scavenge the liberated benzyl carbenium ion." ... [Pg.90]

BF3-Et20, NaCNBHs, THF, reflux 4-24 h, 65-98% yield. Functional groups such aryl ketones and nitro compounds are reduced and electron-rich phenols tend to be alkylated with the released benzyl carbenium ion. The use of BF3 Et20 and triethylsilane as a cation scavenger is also effective." ... [Pg.90]

These examples serve to illustrate the fact that, in reactions in which carbenium ions are formed in proximity to the acetal lone pairs, unexpected rearrangements may occur. [Pg.209]

Forty years after the initial proposal, Sweet and Fissekis proposed a more detailed pathway involving a carbenium ion species. According to these authors the first step involved an aldol condensation between ethyl acetoacetate (6) and benzaldehyde (5) to deliver the aldol adduct 11. Subsequent dehydration of 11 furnished the key carbenium ion 12 which was in equilibrium with enone 13. Nucleophilic attack of 12 by urea then delivered ureide 14. Intramolecular cyclization produced a hemiaminal which underwent dehydration to afford dihydropyrimidinone 15. These authors demonstrated that the carbenium species was viable through synthesis. After enone 13 was synthesized, it was allowed to react with N-methyl urea to deliver the mono-N-methylated derivative of DHPM 15. [Pg.510]

The mechanism was then reexamined 25 years later in 1997 by Kappe. Kappe used H and C spectroscopy to support the argument that the key intermediate in the Biginelli reaction was iminium species 16. In the event, 5 reacted with 3a to form an intermediate hemiaminal 17 which subsequently dehydrated to deliver 16. Iminium cation 16 then reacted with 6 to give 14, which underwent facile cyclodehydration to give 15. Kappe also noted that in the absence of 6, bisureide 8 was afforded as a consequence of nueleophilic attack of 16 by urea (3a). This discovery confirmed the conclusion of Folkers and Johnson in 1933. As far as the proposal from 25 years earlier by Sweet and Fissekis, Kappe saw no evidenee by H and NMR spectroscopy that a carbenium ion was a required species in the Biginelli reaetion. When benzaldehyde (5) and ethyl... [Pg.510]


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A-Silyl-substituted carbenium ions

Acid-catalyzed reactions, transition states carbenium ions

Additions via Carbenium Ion Intermediates

Alkenyl carbenium ions

Alkyl carbenium ions

Alkylation due to Carbenium Ion Formation during Acidolysis

Benzhydryl carbenium ions

Benzhydryl carbenium ions substituted

Carbanion carbenium ion

Carbenes, carbenium ions

Carbenium

Carbenium Ions and Alkoxides

Carbenium ion addition

Carbenium ion center

Carbenium ion mechanism

Carbenium ion salts

Carbenium ion-type mechanism

Carbenium ions 1.2] -rearrangements

Carbenium ions 3-scission

Carbenium ions 7-substituted

Carbenium ions Friedel-Crafts reaction

Carbenium ions addition reactions

Carbenium ions alkenyl cations

Carbenium ions alkyl cations

Carbenium ions alkylation

Carbenium ions alkynyl

Carbenium ions allylic

Carbenium ions as intermediates

Carbenium ions benzylic

Carbenium ions chemical shift tensors

Carbenium ions chemical shifts

Carbenium ions classes

Carbenium ions controversy

Carbenium ions cycloaddition reactions

Carbenium ions cyclopropylmethyl

Carbenium ions disproportionation

Carbenium ions effects, positive charge interaction

Carbenium ions electron-transfer equilibria

Carbenium ions esters

Carbenium ions ethyl

Carbenium ions formation

Carbenium ions halides

Carbenium ions heteroatom-stabilized

Carbenium ions hydride affinity

Carbenium ions in zeolites

Carbenium ions initiation mechanisms

Carbenium ions intermediates

Carbenium ions isomerization

Carbenium ions isopropyl

Carbenium ions methyl

Carbenium ions nonclassical

Carbenium ions nucleophilic substitution

Carbenium ions phenyl

Carbenium ions positive charge substituents

Carbenium ions properties

Carbenium ions reactions

Carbenium ions reduction

Carbenium ions resonance stabilization

Carbenium ions silyl-substituted

Carbenium ions stability

Carbenium ions superacids

Carbenium ions transfer

Carbenium ions transition states

Carbenium ions triaryl

Carbenium ions triphenylmethyl

Carbenium ions trivalent, stabilization

Carbenium ions vinyl

Carbenium ions, /3-cleavage

Carbenium ions, acid catalysis

Carbenium ions, carbocation

Carbenium ions, carbocation classification

Carbenium ions, deprotonation

Carbenium ions, hardness

Carbenium ions, superelectrophilic

Carbenium-iminium ions

Carbenium-oxonium ion equilibria

Carbocations carbenium ions

Carbonium and carbenium ions

Concentration of carbenium ion

Conjugated diene complexes of carbenium ions

Crowded carbenium ions

Cycloalkyl carbenium ions

Evidence for the Existence of Carbenium Ions by Trapping Experiments

Experimental Evidence of Reactive Carbenium Ions

Ferrocene ability to stabilize a carbenium ion

Formation of Surface Alkoxy Species with Carbenium-Ion-Like Properties

Glycosyl carbenium ions

Hydroxy carbenium ion

Interpretation carbenium ions formation

Isolated carbenium ions

Molecular orbitals carbenium ions

NMMO-derived carbenium-iminium ion

Oxygen Versus Sulfur Stabilization of Carbenium Ions

Persistent Carbenium Ions in Zeolites Characterized by NMR Spectroscopy

Planar carbenium ion

Primary carbenium ion

Propyl carbenium ion

Reactions of carbenium ions

Reactions with Protic Acids and Carbenium ions

Rearrangements of Carbenium Ions

Ritter reaction carbenium ion source

Shifts in Carbenium Ions

Solid interaction with carbenium ions

Sp2-hybridized carbenium ion

Stability of Carbenium Ions in Zeolites

Stable carbenium ion

Surface alkoxy species, with carbenium-ion-like

Surface alkoxy species, with carbenium-ion-like properties

Tertiary carbenium ion

Triplet carbenes Carbenium ions

Trivalent-Tricoordinate (Classical) Carbenium Ions

Zeolites alkyl carbenium ions

Zeolites carbenium ions

Zeolites interaction with carbenium ions

Zeolites persistent carbenium ions

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